1. Field of the Invention
The present invention involves techniques relating to a display device comprising an EL (electro luminescence) element (hereinafter referred to as an EL display device) and to a display that uses the EL display device.
2. Description of the Related Art
Studies of EL display devices comprising EL elements as self-light emitting elements have flourished in recent years. In particular, organic EL display devices using organic materials for EL materials have held the attention. The organic EL display devices are also called organic EL displays (OELDs) or organic Light Emitting Diodes (OLEDs).
EL display devices are of self-light emitting type, unlike liquid crystal display devices, and therefore the view angle does not matter in the EL display devices, which forms one of their characteristics. That is, the EL display devices are more suitable for displays used outdoors than the liquid crystal display devices, presenting so many possible ways of their use.
EL elements have the structure in which one EL layer is sandwiched between a pair of electrodes. An EL layer usually has a layer structure. As a typical example thereof, a layer structure consisting of hole transmitting layer/light emitting layer/electron transmitting layer which has proposed by Tang, et al. from Kodak Eastman company can be named. This is so highly efficient in terms of light emission that most of EL display devices whose research and development is now under way employ that structure.
Light is emitted by applying a given voltage generated between the pair of electrodes to the EL layer having the above structure to cause re-combination of carriers in the light emitting layer. A method to achieve this is chosen from two options in which one is to form the EL layer between two kinds of stripe-like electrodes arranged perpendicular to each other (simple matrix method) and the other is to form the EL layer between pixel electrodes that are connected to TFTs and are arranged in matrix and opposite electrodes (active matrix method).
Both methods requires to form the EL layer on the electrodes that have been patterned out. The EL layer, however, is easily influenced and degraded by changes such as level differences, so that various kinds of contrivance are made to improve the flatness. This leads into a problem of complication of manufacturing process and, accordingly, an increase in production cost.